Process for producing steel products having a hydrated chromium oxide film excellent in the surface appearance

ABSTRACT

A process for obtaining a hydrated chromium oxide treated steel article having a beautiful surface appearance without any roll marks by pressing a steel product subjected to the chromic acid cathode electrolytic treatment, while a chromic acid film thus electrodeposited on the surface of the steel product is still wet, immediately after the electrolytic treatment by the help of a leveling roll as an auxiliary holddown roll having a surface hardness of 40* to 90* (A scale) without need of frequently exchanging a holddown roll installed for preventing the formation of arc spots.

United States Patent Inventors Akira Hata;

Tamotu Hinami, both of Kitakyushu, Japan Appl. No. 809,891 Filed Mar.24, 1969 Patented Oct. 26, 1971 Assignee Nippon Steel Corporation Tokyo,Japan Priority Mar. 28, 1968 Japan 43/ 197 17 PROCESS FOR PRODUCINGSTEEL PRODUCTS HAVING A HYDRATED CHROMIUM OXIDE FILM EXCELLENT IN THESURFACE APPEARANCE 7 Claims, 1 Drawing Fig.

US. Cl 204/35 N Int. Cl C23b 11/00 Field of Search 204/35, 36,

[56] References Cited UNITED STATES PATENTS 488,499 12/1892 Hollingshead204/36 1,488,553 4/1924 Peacock 204/35 Primary Examinerl-loward S.Williams Assistant Examiner-W, 1. Solomon Attorney-Wenderoth, Lind &Ponack ABSTRACT: A process for obtaining a hydrated chromium oxidetreated steel article having a beautiful surface appearance without anyroll marks by pressing a steel product subjected to the chromic acidcathode electrolytic treatment, while a chromic acid film thuselectrodeposited on the surface of the steel product is still wet,immediately after the electrolytic treatment by the help of a levelingroll as an auxiliary holddown roll having a surface hardness of 40 to 90(A scale) without need of frequently exchanging a holddown rollinstalled for preventing the formation of arc spots.

PATENTEDUET 2B |97l 3,616.2 9 9 INVENTOR Akiro Hafo Tomofu Hinami 1 A BYZUQMJJLM ATTORNEY PROCESS FOR PRODUCING STEEL PRODUCTS HAVING A HYDRATEDCHROMIUM OXIDE FILM EXCELLENT IN THE SURFACE APPEARANCE This inventionrelates to a process for producing steel products having a film ofhydrated chromium oxide excellent in the surface appearance by a chromicacid cathodic electrolysis treatment.

In order to electrolyze hydrated chromium oxide onto the surface of aniron or steel product, there is usually practiced a method in which suchmaterial to be treated as, for example, a properly pretreated steelplate is electrolytically treated in a treating bath mainly composed ofsuch hexavalent chromium ions as chromic acid ions or dichromic acidions, and additionally sulfuric acid or another inorganic acid to beadded, if required.

Due to excellent properties of films formed by the chromic acid seriestreatment as above-mentioned, particularly in the anticorrosion andpaint adhesion, an increasing importance has been attached to the saidtreatment as an antirusting treatment of iron and steel products.However, chromic acid series films, especially the hydrated chromiumoxide film obtained by the above-mentioned treatment, have adisadvantage that they are so soft as to be likely to be flawedparticularly by rolls when they are still wet immediately afterelectrodeposition.

In addition thereto, since an anticorrosion film obtained by a cathodictreatment with an electrolyte containing mostly chromic acid ions has anelectric insulating property, there occur often are spots particularlyon conductor rolls.

The above-mentioned problems of the chromic acid series coated surfaceformed by a cathodic treatment shall be explained with reference to adrawing.

The drawing is a schematic view of an improved electrolysis treatmentsection for working the present invention.

This electrolysis treatment section is connected with degreasing,water-rinsing and pickling steps as the preliminary stage thereof and isfollowed by a heating and drying step as the latter stage thereof.

As compared with a known electrolysis treatment section the improvementof the present invention is characterized only by providing a levelingroll 7 as an auxiliary holddown roll in such manner as shown in thedrawing, but is quite the same as the former in other respects.

in the known electrolysis treatment a steel plate is led with conductorrolls 4 and 4' which are provided with holddown rolls 3 and 3respectively, and with sink rolls 5 which are installed as dipped in anelectrolysis bathtub 2, in which there is held a treating solutioncontaining water-soluble hexavalent chromium ions, and meanwhile arequired film is formed on the surface of the steel plate. 6 is an anodeprovided as dipped in the electrolyte and opposed to the surface of thepassing plate.

Heretofore, when carrying out an electrodepositing treatment with knownequipments a nonuniform contact was unavoidably formed between the steelplate and the conductor rolls 4 and 4', whereby an electric dischargeoccurred between them, resulting in the production of perforations"called arc spots or discharge traces on the surface of the steel plate.The holddown rolls 3 and 3 attached to the conductor rolls 4 and 4'respectively are originally provided for the purpose of attainingsufficient contact of the steel plate 1 with the conductor rolls 4 and4'. These holddown rolls 3 and 3 are coated with material which has ahigh hardness above 90in the Shore Durometer hardness (A scale) and isdifficult to elastically deform as, for example, a hard vinyl chloride,or may be coated with an elastic material ofa comparatively low hardnessas of 40to 90in the Shore Durometer hardness (A scale) so that thepressing force may be elevated and the contact between the steel plateand conductor rolls may be improved to prevent the production of arcspots.

However, the arrangement ofthe holddown rolls 3 and 3' asabove-mentioned is attended by the following difficulties:

As the surface layer of the anticorrosion film electrodeposited on thesteel plate, which is made a cathode, is made of hydrated chromium oxidein the form ofa sol, it is so soft that, if the surface of thecontacting roll is defective, there is produced an unevenness incoloring, or else the electrodeposited film is easily scraped off androll marks are left on the film surface while the steel plate isadvancing.

In the case of using the holddown rolls 3 and 3 made of a materialhaving a high hardness the steel plate must suffer from a deteriorationof the surface flatness as a result of the fluctuation of the tension ofthe steel plate during its passage at a high speed as, for example,about 200 to 290 m./min. That is, there appears a phenomenon of thesteel plate being drawn by deformation, whereby the surface of theholddown rolls are flawed, again causing the soft electrodeposited filmto scrape off by irregular flaws thus formed on the roll surface.

Heretofore, when the holddown roll was thus flawed, the flawed roll wasimmediately replaced by a new one. This replacement was, however,attended with various disadvantages, for instance, the line had to bestopped and the frequent replacement resulted not only in complicatingthe operation, but also in remarkably reducing the operation efficiency.

Further, a special consideration was required for the sake of the liningmaterials of the rolls, because the treating solution was mainlycomposed of chromic acid series chemical having a strong oxidizingproperty. Consequently, a higher cost had to be paid for the liningmaterials.

in view of these facts as above-mentioned the present inventors havemade various investigations to eliminate such disadvantages and haveconfirmed as the result of the investigations that it is almostimpossible to obtain the treated steel plate having a uniform film onits surface when selecting the elastic material for lining the surfaceof the holddown rollfrom those comparatively low in hardness to elevatethe pressing force only, because in this case the holddown roll islargely elastically deformed and the contact area between the steelplate and the holddown roll surface expanded, whereby the formedcorrosion-resisting film is often easily scraped off, but that thecorrosion-resisting film on the side in contact with the conductor rollis substantially not flawed, irrespective of the hardness of the surfacelining material of the holddown roll, because the conductor roll isplated on the surface with hard chromium.

As regards tin-free steels practiced at present as steel plates formaking cans those having chromium series coatings occupy almost themonopolistic position, and it is thought that, when the coating is ofatwo-layer structure made ofa metallic chromium inner layer high in theanticorrosion and a nonmetallic chromium outer layer, said coating wouldbe superior to a coating made of a single layer of metallic chromium inthe paint adhesiveness, workability, productivity, electrodepositingefficiency at the time of electrodepositing the coating and otherrespects. However, it is to be mentioned as a disadvantage of thecoating of a two-layer structure that on the nonmetallic chromium outerlayer, for example, a hydrated chromium oxide layer of a plating amountof 0.l to 2.5 mg./dm. or preferably 0.2 to 0.6 mg./dm. the colorirregularity due to the nonuniformity of the plating amount or socalledroll mark is conspicuous on account of the presence of the inner layerof metallic chromium of a plating amount of 0.5 to 2 mg./dm."' having awhite metallic lustrous surface, which is not desirable from theviewpoint of the commodity appearance.

The object of the present invention is to provide a method ofcontinuously electrolytically treating steel plates to fonn thereon asurface layer of a hydrated chromium oxide film having a beautiful anduniform surface by erasing flaws or holddown roll marks unavoidablyproduced on the electrodeposited film surface owing to such flawedholddown rolls as is mentioned above.

That is, according to the present invention flaws on the film surfacemay be erased by pressing the film surface on the side of the holddownroll 3' using a leveling roll (in the drawing this roll is indicated bya number 7), said leveling roll being lined with an elastic materialhaving a surface hardness of 40to in the Shore Durometer (A scale) andbeing provided in the rear of the holddown roll 3 a hardness of morethan 90or generally lOin the Shore Durometer (A scale), said holddownroll 3' being provided as attached to the final conductor roll 4' on theperipheral surface thereof, the steel plate 1 passing between saidholddown roll 3' and the final conductor roll 4' after having finallyleft the electrolytic bath 2, or being provided as a roll separate fromthe above-mentioned conductor roll between said conductor roll 4 and afilm heating and drying section (not illustrated).

In other words, in the present invention the leveling roll 7 is providedon the final conductor roll 4 as is shown in the drawing or is providedin the part within drag-out tanks No. l, 2 ..arranged in connection withthe latter part of the electrolytic treating section but so adjacent tothe electrolytic treating section as possible, or a deflector roll ofthe said dragout tanks is utilized as a leveling roll, though notillustrated. in the above-mentioned case the hardness of the elasticmember applied to line the surface of the leveling roll is 40to 90in theShore Durometer (A scale), that is, lower than that of theabove-described holddown roll. Needless to say, the position of fittingthe leveling roll is not limited to only those mentioned above, but anyeffective position may be found within the range covering from the finalconductor roll to the heating and drying section. Further, it is to benoted, that at least either one of the leveling rolls, when they arearranged in a pair, must have a surface of an elastic member.

in the present invention, the steel plate means a cold-rolled steelsheet, chromium-plated steel plate, zinc-plated steel plate, tin-platedsteel plate or any other iron or steel product, a continuous steel stripof a thickness of about'O. l 2 to 1.2 mm. being preferred.

it is most favorable not only from the viewpoint of the equipment butalso from that of the operation to provide the leveling roll 7 in therear of the holddown roll 3' so as to be opposed to the surface of theconductor roll 4' where the film on the steel plate 1 is still wet justafter it is electrodeposited until it is heated and dried. However, solong as the film on the steel plate having passed the holddown roll 3'is wet, the leveling roll need not always be opposed to the conductorroll 4' and can be provided independently in the rear of the conductorroll 4' together with an opposed roll (not illustrated).

Further, it is necessary to adapt the pressing force of the levelingroll 7 to the thickness of the film. According to the finding by thepresent inventors, a favorable result can be obtained with a pressingforce of less than 30 kgjcrn? or preferably 4 to 6 kg./cm.

In the present invention, the leveling roll 7 is made to act on theelectrodeposited film while it is still wet. Once the film dries, itbecomes very difficult to erase the flaws made on the film surface.

The effect of leveling the film surface with the leveling roll 7 canalso be adjusted by regulating the wet state of the film surface byapplying such means as spraying the electrolyte onto the film surfacebetween the holddown roll 3' and leveling roll 7.

The leveling roll 7 is desirably to be of a hardness of 40 to 90 orpreferably 60 to 75 in the Shore Durometer hardness on the A scale.

As the treating bath relates to such Cr -containing aqueous solutionshaving strong oxidizing property as, for example, an aqueous solutioncontaining dichromic acid ions or chromic acid ions, the elasticmaterial is selected from a group consisting of a chromic acidproofrubber, chloroprene rubber, polypropylene, vinylidene chloride,chlorinated polyethylenevinyl chloride and chlorosulfonated polyethyleneresin which will not deteriorate in contact with said solution. However,when taking the rollability, heatproofness and treating bathcontaminating property into consideration, a roll lined with a rubbermaterial made mainly of a chlorinated polyethylene resin is mostpreferable.

As described above, the surface hardness of the leveling roll is limitedto within the range of 40 to 90 in the Shore Durometer hardness on the Ascale. This is because, when the hardness is more than the roll surfacewill be too hard and will be likely to be flawed and when the hardnessis less than 40 it will be difficult to erase flaws on the film surface.

If this leveling roll 7 is arranged in the rear of the holddown roll 3or 3 so that it may work together with said'holddown roll 3 or 3 linedfor example, with a hard vinyl chloride resin of a high hardness of morethan 90 in the Shore Durometer hardness on the A scale, it is possibleto obtain a steel plate having a uniform smooth anticorrosion film freefrom are spots and roll marks or scratches.

The surfaces of the rolls located in the plate passing section from thefinal conductor roll to the heating and drying step should have ahardness which is the same as or lower than the hardness of theabove-described leveling roll. But their pressing force can be higherthan that of said leveling roll in some cases.

An example of the present invention shall be described in the following.However, the object of the present invention is not limited to thehydrated chromium oxide coated steel plate as shown in the followingexample, but also includes chromic acid coated materials made with anyother bath composition, and the means of erasing roll marks in thechromium oxide coating, and in addition thereto the position of settingthe leveling roll, the material for lining the leveling roll, thepressing force and others may be altered freely within the scope of theclaims.

EXAMPLE In an electrolytic treating apparatus, in which holddown rolls,conductor rolls, dipping rolls and a leveling roll are arranged as isshown in the drawing, the hardness of the surface of the holddown rollwas made l00 in the Shore Durometer hardness (A scale), the hardness ofthe surface of the leveling roll was made 65 also in the Shore Durometerhardness (A scale) and a steel strip on the final conductor roll waspressed with a pressing force of IOkg/cm. by the leveling roll.

The electrolytic treating solution was an aqueous solution of 45g./l. ofchromic anhydride and 0.4g./l. of sulfuric acid at a bath temperature of55C. The above-mentioned steel strip was electrolytically treated as acathode at a current density of 30 amp./dm. for 3seconds and wasadvanced at a high speed through the above-mentioned final conductorroll, drag-out tanks and wringer rolls to the heating and drying step.The steel plate was water-washed and was then dried to obtain a producthaving a uniform electrodeposited film rich in luster and having nocolor irregularity, roll mark or coating defect.

What is claimed is:

l. A process for producing steel products having a hydrated chromiumoxide film excellent in the surface appearance by electrodepositing ananticorrosion film by electrolytically treating an iron or steel productas a cathode in an electrolyte containing hexavalent chromium ions,comprising a step of pressing the electrodeposited film surface with aleveling roll of a surface hardness of 40 to 90 in the Shore Durometerhardness (A Scale) while said electrodeposited film is still wet afterthe steel product coated with the electrodeposited film has passed aholddown roll provided on a final conductor roll.

2. A process according to claim 1 wherein said iron or steel product isa continuous cold-rolled steel sheet, steel plate, chromium-plated steelplate, zinc-plated steel plate or tinplated steel plate.

3. A process according to claim 1 wherein said leveling roll is linedwith a chromic acid-proof elastic rubber or resin.

4. A process according to claim 3, wherein said iron or steel product isa continuous cold rolled steel sheet, steel plate. chromium-plated steelplate, zinc-plated steel plate or tinplated steel plate.

5. A process for producing steel products having a hydrated chromiumoxide film excellent in the surface appearance by electrodepositing ananticorrosion film by electrolytically treating an iron or steel productas a cathode in an electrolyte containing hexavalent chromium ions,comprising a step of pressing the electrodeposited film surface with aleveling roll of a surface hardness of 40 to 90 in the Shore Durometerhardness (A scale) under a maximum pressure of 30kg./cm." while saidelectrodeposited film is still wet after the steel product coated withthe electrodeposited film has passed a 5 holddown roll provided on afinal conductor roll.

6. A process according to claim 5, wherein said leveling roll I e ll

2. A process according to claim 1, wherein said iron or steel product isa continuous cold-rolled steel sheet, steel plate, chromium-plated steelplate, zinc-plated steel plate or tin-plated steel plate.
 3. A processaccording to claim 1, wherein said leveling roll is lined with a chromicacid-proof elastic rubber or resin.
 4. A process according to claim 3,wherein said iron or steel product is a continuous cold-rolled steelsheet, steel plate, chromium-plated steel plate, zinc-plated steel plateor tin-plated steel plate.
 5. A process for producing steel productshaving a hydrated chromium oxide film excellent in the surfaceappearance by electrodepositing an anticorrosion film byelectrolytically treating an iron or steel product as a cathode in anelectrolyte containing hexavalent chromium ions, comprising a step ofpressing the electrodeposited film surface with a leveling roll of asurface hardness of 40* to 90* in the Shore Durometer hardness (A scale)under a maximum pressure of 30kg./cm.2 while said electrodeposited filmis still wet after the steel product coated with the electrodepositedfilm has passed a holddown roll provided on a final conductor roll.
 6. Aprocess according to claim 5, wherein said leveling roll is lined with achromic acidproof elastic rubber or resin.
 7. A process according toclaim 5, wherein said iron or steel product is a continuous cold-rolledsteel sheet, steel plate, chromium-plated steel plate, zinc-plated steelplate or tin-plated steel plate.